should I be removing this?

[cspcrx]

I am sure this question shows my lack of understanding around welding and metal working. In another thread I posted some pictures of my first welds. The question is should I be removing the black coating off of the plate before I weld it? I tried with my flap disk and it seems like to get it off of the edges I had to removed actual material. You can see that at the edges of the plate in this picture. I just removed the paint and ran it across the surface, but it was not bright and shinny like it is on the edges. Thank you all for helping the ignorant.

 

[MoonRise]

That 'black stuff' is "mill scale".

It is a form of iron oxide.

It is not 'metal', it is a metal oxide.

When welding, you are welding metal.

Clean and shiny metal is what you want to weld.

[note and disclaimer: Some welding processes can 'tolerate' certain amounts of 'non-metal' (mostly the flux containing process such as SMAW aka stick or FCAW), some process can tolerate a little bit of 'non-metal' (GMAW with a relatively high amount of deoxidizers such as an S6 wire compared to an S2 wire), and some process will tolerate almost no 'non-metals' (such as GTAW aka TIG). ]

But you will ALWAYS get a better weld if you are welding (or attempting to weld) clean bright shiny metal.

Not rust or millscale or any other oxides, no grease or oil or paint, etc, etc.

IMNSHO.

further note: On steel, a flap disk or hard grinder wheel is needed to actually remove mill scale. A wire brush (hand or on an angle grinder) doesn't usually remove mill scale, it just sort of 'cleans' it and polishes it a bit. Use a flap disk or grinder wheel.

 

[APEowner]

MoodRise pretty much said it all. I will admit to MIG and stick welding right over mill scale (as well as paint and outright rust) in some no-critical applications but I'd never claim that was correct.

 

[Strouty]

I can remember using 6011, nothing would stop that from sticking metal together.

 

[383 240z]

Try some cold rolled steel. No mill scale on that. I usually toss my stuff in the blaster to get the scale off. Then it's just a matter of getting the glass off the material. Keith

 

[csp]

Pickled and oiled also has no mill scale and it's cheaper than cold rolled.

These work excellent for removing mill scale. Even better than a flap disc, but they don't wear very well.

 

[dv8customs]

I agree, those stripping discs work very well on mil scale

 

[dr_clyde]

Walter abrasives also makes a disc specifically for mill scale.

http://www.walter.com/en_US/products/abrasives/blending/flexcut-mill-scale

 

[Strouty]

Walter abrasives also makes a disc specifically for mill scale.

http://www.walter.com/en_US/products/abrasives/blending/flexcut-mill-scale

Have you used them? If so do you like them?

 

[dr_clyde]

I have used them, and they work well. I don't use a ton of hot rolled so I'm not running these all day.

But for the odd piece of steel I have yet to find something that works as good as these.

 

[Strouty]

I rarely use cold rolled. Usually my steel has rust on it. Mill scale has always been an issue for me. Probably the labor to clean it is equal to buying cold rolled.

 

[kazlx]

Mill scale is such a pain in the ***, that I won't use it unless I absolutely can't get cold rolled.

 

[Strouty]

How much more are you paying for cold rolled? Does it rust quicker?

 

[Kevin54]

Mill scale is usually on hot rolled steel, and is a pain. Plus hot rolled is not as square as cold rolled steel. I'm not a welder by a longshot, but I do know that the welders at the shop would remove all of the mill scale in the area where they were welding. I would grind it all off before welding to make sure you get good penetration.

 

[kazlx]

I think anything over 1/4" plate is HRS, or at least what I have access to here. You can also get P&O. The prices aren't that much more expensive. New steel is around $1/lb and I can get it cheaper when I buy 250-500lbs at a time, which isn't hard when I buy sheets for the plasma. There has been a few times, it would cost me like $200 for two sheets or $50-60 more for 4 sheets. No brainer. With the plasma running, sheet goes fast around here. I pretty much refuse to use angle iron because of the scale and I think it looks cheesy. They do have thinner CRS angle at my supplier, I think down to 14g.

On CRS I get, whatever I have left that I don't use right away, I usually just wipe down with WD-40 and it's good for a while, but I live in SoCal and moisture isn't a huge problem.

 

[cspcrx]

WOW thanks everyone for the feedback! It is a pain in the **** to get off with my flap disk. I will try different steel and also the orange disk.

I really appreciate you sharing you knowledge.

 

[Strouty]

WOW thanks everyone for the feedback! It is a pain in the **** to get off with my flap disk. I will try different steel and also the orange disk.

I really appreciate you sharing you knowledge.

Some of us appreciate the questions, otherwise these answers would have never been shared.

I never really thought much about mill scale, as far as cold rolled, I don't think I have ever bought it. I usually buy hot rolled and just grind it. So when you guys use cold rolled, are you painting it after the project is done? I know the mill scale does a pretty decent job of keeping the rust down.

 

[mike13u]

For a quieter and less messy alternative to the above, put a little Naval Jelly on the joint for about 15 minutes. Wipe off with a wet rag. Then, hit the seam with a wire brush and its ready to go.
I only remove mill scale where I need to if painting or patina. If it all has to come off and I dont have cold rolled it gets an acid bath.
And yes, it always has to come off the weld area. Get into the habit of it now. It will make your welds better. Like painting, its all about the prep.

 

[Strouty]

Cool tip with the naval jelly.

 

[HMCFab9]

Vinegar is supposed to remove it. Muriatic acid does too, but that's Nasty stuff!

 

[kazlx]

I like the strip wheels. They seem to take off the scale and not really affect the base metal.

 

[CGT80]

That 'black stuff' is "mill scale".

It is a form of iron oxide.

It is not 'metal', it is a metal oxide.

When welding, you are welding metal.

Clean and shiny metal is what you want to weld.

[note and disclaimer: Some welding processes can 'tolerate' certain amounts of 'non-metal' (mostly the flux containing process such as SMAW aka stick or FCAW), some process can tolerate a little bit of 'non-metal' (GMAW with a relatively high amount of deoxidizers such as an S6 wire compared to an S2 wire), and some process will tolerate almost no 'non-metals' (such as GTAW aka TIG). ]

But you will ALWAYS get a better weld if you are welding (or attempting to weld) clean bright shiny metal.

Not rust or millscale or any other oxides, no grease or oil or paint, etc, etc.

IMNSHO.

further note: On steel, a flap disk or hard grinder wheel is needed to actually remove mill scale. A wire brush (hand or on an angle grinder) doesn't usually remove mill scale, it just sort of 'cleans' it and polishes it a bit. Use a flap disk or grinder wheel.

Well said, and even covered the S2 vs. S6 welding wire.

Mill scale and rust ****. I live in SoCal, so it isn't anything like what I see others put up with. For quick jobs that don't matter (such as mocking up a part or a temporary jig) I might skip grinding. When I want good welding results, I grind, blast, or anything else needed to get bright metal. When you work with aluminum and tig welding, it is a must.

Most people think of welding as a dirty job, but the work actually needs to be clean, depending on what you want out of the welds.

 

[cspcrx]

I am using S6 wire which may be why I was able to get away with the welds I did. I appreciate the feedback and help from everyone. Learning a lot.

 

[csp]

Since cold rolled keeps being mentioned I'll bring it up again that pickled and oiled steel also is free of mill scale. It's cheaper than cold rolled as well.

P&O is just hot rolled dropped into a bath of "pickling" (usually an acid) to remove the mill scale and then very lightly oiled to keep it from rusting.